In the existent ball screw device of this type, a nut having a thread groove at the inner circumferential surface is screw coupled with the screw shaft having a thread groove at the outer circumferential surface and extending in the axial direction, the thread groove corresponding to the thread groove of screw shaft, in which the thread groove of the nut and the thread groove of the screw shaft are opposed to each other to define a spiral load raceway between both of them. A plurality of balls loaded rollably as rolling elements along the load raceway, and the nut (or screw shaft) moves axially by the rotation of the screw shaft (or nut) by way of rolling movement of the balls.
By the way, when the nut (or screw shaft) moves in the axial direction, the balls move under rolling along the spiral load raceway formed between both of the thread grooves, and it is necessary to infinitely circulate the balls in order to continuously move the nut (or the screw shaft).
As a system for infinite circulation of the balls, a circulation tube type, an end cap type or the like is general. In the compact type ball screw device, an axially penetrating ball return passage is formed to the nut, a recess to which the end of the ball return passage opens is formed to the end of the nut, and a circulation block having a ball circulation groove is fitted and secured to the recess for communication ball return channel and the load raceway between both of the thread groove, thereby circulating the balls infinitely.
By the way, while a ball running surface of a width larger than the ball diameter for constituting, together with the ball circulation groove in the circulation block, a ball circulation passage is provided to the recess formed to the end surface of the nut, the ball running surfaces is used as an inner surface directing the radial direction of the recess and, accordingly, the ball circulation groove of the circulation block is also formed to the radially directing surface so as to oppose to the ball running surface, and thick walled portions are disposed to both axial sides of the ball circulation groove respectively. However, in a case where the thick walled portions are disposed on both axial sides of the ball circulation groove of the circulation block, in a ball screw device with a small lead screw with narrow inter thread groove distance or a ball screw device of multi thread screw, a portion of the recess of the nut corresponding to the thick walled portions in the axial inside of the ball circulation groove may sometimes interfere the adjacent thread grooves (on the side of nut), to provide a disadvantage not capable of coping with the improvement of the load capacity by the increase in the size of the ball diameter and the number of threads.
Further, in the circulation block, while the block main body having the ball circulation groove is generally provided with a scoop-up portion of scooping up the balls rolling between both of the thread grooves to the ball circulation passage integrally, since the scoop-up portion is inserted and arranged to the thread groove of the screw shaft, in a case where the thread groove for the screw shaft is cut leaving the axial end (in a case where the thread groove is not formed as far as the end face of the screw shaft), it results in a disadvantage that the circulation block can not be mounted to the recess of the nut in a state of inserting the nut over the screw shaft, making it impossible for assembling the ball screw device.
The present invention has been accomplished for overcoming such a disadvantage and a first object thereof is to provide a ball screw device capable of improving the load capacity by increasing the size of the ball diameter and the number of threads by preventing the recess of the nut from interfering thread grooves adjacent to the recess even in the thread groove of a small lead screw and a multiple thread screw.
A second object of the present invention is to provide a ball screw device capable of easily mounting a circulation block to the recess of the nut in a state of inserting the nut over a screw shaft even in a case where the thread groove of the screw shaft is cut leaving the axial end.